Homopolymers and copolymers of vinyl chloride are among the most important plastics, or polymeric compositions, of commerce. These polymers have been produced in significant and increasing quantities for over thirty years, and currently are manufactured at a rate exceeding five billion pounds annually.
Vinyl chloride polymers are made by a number of processes, including emulsion, suspension, solution, and bulk. Each of these processes has achieved some degree of commercial success, the choice of process being largely determined by the desired characteristics of the polymer, and each is capable of an economically acceptable degree of conversion of monomer to polymer. However, the polymerization processes as actually practised commercially result in polymers having a high content of vinyl chloride monomer ranging, for example, from 100 to 15,000 or more parts of monomer per million of polymer (weight/weight). Such high monomer content resins, or polymers, have heretofore been technically and economically acceptable to the art. Both monomer and polymer are relatively low in cost, and it had been believed until recently that vinyl chloride monomer was essentially physiologically harmless. Further, since these polymers are generally fabricated into useful compounds, shapes, and articles by heating them at elevated temperatures ranging from about 125.degree. C. to 200.degree. C. or higher, it had been assumed that residual monomer was driven off by such heat treatment since vinyl chloride monomer boils at -13.4.degree. C. Hence, there has been no particular concern over the amount of free VCM in polymers and little or no effort has been expended in reducing this amount, until recently.
Quite surprisingly, in view of the fact that VCM is a gas at ordinary temperatures and pressures, boiling at -13.4.degree. C., it has been found to be tenaciously held in the free and unreacted state in polymers, even after the polymers have been processed and fabricated at elevated temperatures. Illustratively, compostions based on vinyl chloride homopolymers or copolymers, plasticized or unplasticized, are generally prepared utilizing various combinations of process steps including blending at elevated temperatures in a ribbon blender or Henschel mixer, extrusion, fusion using a two-roll mill, fusion using a Banbury or other intensive mixer, calendering, and press-polishing. Temperatures commonly employed for extrusion, two-roll mill or intensive mixer fusion, calendering, and press-polishing range from about 170.degree. C. to 200.degree. C. or even higher, and the time at which the polymer compositions are held at these elevated temperatures is approximately the same as the time required to remove the VCM in the method of the present invention. Despite the elevated temperatures employed, only a portion of the VCM, approximately 50% or less, originally present is removed during a typical processing cycle for a rigid polyvinyl chloride composition. It has not been possible to achieve a non-detectable VCM level in such compositions using conventional processing cycles and commercial polymers having the usual amounts of VCM. This has resulted in polyvinyl chloride (PVC) being judged unacceptable for certain uses for which it is otherwise eminently suitable. For example, alcoholic beverages contained in bottles made of PVC have been discovered to be contaminated with vinyl chloride monomer extracted from the bottle wall. As a consequence, PVC bottles have been disallowed for packaging such beverages. Although PVC is know to be thermally unstable, no evidence exists to establish that VCM is generated as a result of such degradation. Studies of PVC degradation by numerous investigators over a period of many years have failed to reveal the presence of vinyl chloride among the decomposition products. Hence, any monomeric vinyl chloride found in polymer must be residual monomer, unconverted during the polymerization process.
A more recent development has caused additional concern about residual VCM levels, and has spurred interest in reducing them. Evidence has been produced which implicates vinyl chloride monomer as one possible cause of a rare liver cancer known as angiosarcoma. Knowledge of this potential health hazard has inspired research into methods for reducing or eliminating the residual VCM in polymers, particularly for polymers to be used in the manufacture of articles for handling and packaging of foods, beverages, cosmetics, drugs, and pharmaceuticals.
To this end, several methods have been proposed, including stripping of aqueous suspensions or emulsions of polymer at temperatures of up to 125.degree. C. during the polymer manufacturing process; adding a volatile organic liquid to such aqueous suspensions or latices of polymer, and then stripping at elevated temperatures; extracting latices of PVC with a liquid hydrocarbon as disclosed in U.S. Pat. No. 3,052,663 to Bodlaender et al; and steaming of wet polymer cake derived from centrifuging of such aqueous suspensions, or latices, of polymer, as disclosed in U.S. Pat. No. 3,956,249 to Goodman et al. Also, German Offenlegungsschrift No. 2,331,895 discloses a process for removing VCM from vinyl chloride polymers by heating the polymer to a temperature between the transition range and 180.degree. C., condensing steam directly onto the polymer, and then cooling the polymer below its transition range by evaporating the steam which has condensed on the polymer.
Although these methods have achieved some degree of success in reducing, or even eliminating, residual VCM in the polymer, none has been completely satisfactory. Each of these prior methods suffers from one or more disadvantages, such as the need to install additional and costly equipment; an increase in the time required to manufacture a given amount of polymer; and the fact that the exposure of the polymer to elevated temperatures for sufficient time to reduce the VCM content to a very low or non-detectable level may cause some degradation and color formation thus rendering the polymer unsuitable for some uses wherein high quality is critical.